WO2010/072198 A1, which is incorporated by reference, describes a method for detecting rain with the aid of a camera, which is used for automotive driver assistance functions. To detect rain, a bifocal optical system is used which produces a sharp image of a partial area of the windscreen on a partial area of the imager or image sensor of the camera. In order to be able to detect raindrops even at night, it is further proposed to couple light into the windscreen using a coupling-in element and to transfer this light within the screen by means of total internal reflection. The light which has been entirely reflected is then coupled out towards the camera by a coupling-out element. When there are drops of water on the windscreen, part of the light is coupled out rather than reflected to the coupling-out element.
U.S. Pat. No. 7,259,367 B2, which is incorporated by reference, also proposes a method for sensing rain by means of a camera, wherein a large area of the window of view, which is defined by the camera's angular aperture and the windowpane, is illuminated. The camera is almost set to infinity focus and can thus be used for driver assistance applications at the same time. As an image of the far range is produced, raindrops can only be noticed as faults in the image, which are detected by complex difference measurements of the images recorded using light that is pulsed or modulated in synchronization with the pixel frequency.